Lucas Magdyno Rebuild

Magnetoman

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greg brillus

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I always found the magneto part of a magdyno seems to have low "Compression" when you rotate the armature, I guess this is due to the early magnet choice/design. Whereas the later K-2F style would have had Alnico type magnets.......Does this sound at all correct.........?
 

nkt267

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I have found that on my earlier Lucas MSL mag/dyno with the open magnets..John
 

Magnetoman

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I always found the magneto part of a magdyno seems to have low "Compression" when you rotate the armature, I guess this is due to the early magnet choice/design. Whereas the later K-2F style would have had Alnico type magnets...
Post-War Magdynos, like the one I just rebuilt, also have Alnico magnets, but I've yet to cross-section one to see if the size of the magnet is the same as in a KVF or KNC1 magneto.

The resistance to turning just before the point of ignition is due to the compression of the field lines from the magnet. A magneto is timed to open its points just as the compression has reached a maximum and they find it energetically favorable to take the alternative path to the other side through the center of the armature. This provides maximum change in field with respect to time (dB/dt) and thus maximum voltage. The fact a fully magnetized Magdyno provides sufficient voltage to spark across 5 mm when turning less than 150 rpm (300 rpm engine) says the Alnico in it is large enough. However, it's still possible it has a smaller magnet, in which case it would supply less current, i.e. less energy to the spark.

It turns out that over the past winter I designed and collected all the components for the Mother of Electrical Testers, and had hoped to have time to start building it by now. When it's done, among other things its VFD-controled variable speed 1/3 h.p. motor will allow me to spin any motorcycle electrical device -- magneto, dynamo, or alternator -- and measure the output. In the case of a dynamo or alternator, it will balance the load against power resistors (up to the max. ~250 W provided by the motor). In the case of a magneto it will measure both the spark current as well as the voltage at any rpm and thus display the spark energy in milliJoules. That will answer the question of whether or not a Magdyno produces a lower energy spark than a, say, KVF. It also will answer the question of how much difference there is in the spark energy provided by Cylinder 1 of a KVF vs. Cylinder 2.

I'd say "stay tuned" for results, but you can safely tune elsewhere until at least the late Fall. It will be that long before I will have a chance to start assembling and, um, ahem, testing the new tester.
 

timetraveller

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I'm never going to rewind a magneto so this question is just a general interest one. Some of the modern rare earth magnets can be very strong. For those who know about this type of thing, would there be any advantage in rebuilding a magneto with rare earth magnets?
 

Magnetoman

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For those who know about this type of thing, would there be any advantage in rebuilding a magneto with rare earth magnets?
The answer is an unequivocal 'yes, but.'

Thanks to advances in magnetic materials the portion of a "Bosch-type" (i.e. Lucas) magneto devoted to the magnet significantly decreased between the time of its invention before WWI and the final production in the early 1960s. This is shown in the side-by-side comparison of the last photograph at:

http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=470880#Post470880

However, over this time magnetos stayed essentially the same size largely because the armature couldn't be reduced much in volume even though the magnets could be, plus the fact the mounting arrangements (e.g. spindle height, size and orientation of mounting holes, etc.) long had been standardized.

To come back to your question, if you could open up the casting of a Lucas magneto you could replace the slug of Alnico with a smaller slug of a rare earth magnet.

However, despite the higher energy and higher remnant magnetic field, if you designed a new rotating armature magneto from scratch you couldn't make the armature very much smaller because of the limitations of the electrical conductivity of the copper needed for the wires. Still, pretending a magneto is a cube for the purposes of making an estimate, if the rare earth magnet allowed you to make each side of the cube 20% smaller you would end up with a magneto occupying only half the former volume. That also means it would be roughly 50% lighter. However, you would have the problem mounting that smaller magneto. In the case of a platform magneto, the standardized spindle height means you would have to give back some of the gains in the form of extra Al to raise it to the required height. But, when you were all done designing and fabricating this new rare earth magneto you would have saved yourself a few pounds but not have something that ignited the fuel any better than a properly-performing 50-year old Lucas. That Lucas produces enough mJ in the spark to fully ignite the mixture and you can't do better than "fully."
 

greg brillus

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The newer generation of magnetos from Joe Hunt and Morris both use rare earth Neodymium magnets in their rotating armature similar in design to a SR 1 Lucas or even the Picador type Scintilla. I know these can put out between 5 to 8 Amps when spinning at higher engine speeds. I suffered an output coil failure on my Joe Hunt on the twin racer which was less than 6 months old, to which I think I found what caused it, the replacement coil has been fine ever since. Some racers have suggested that the output from the newer magnetos is too much for the electrics to cope.......I am inclined to believe that to some extent, but I feel Magnetoman's test rig would certainly be an optimum way of proving that theory.
 

Magnetoman

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... rare earth Neodymium magnets in their rotating armature similar in design to a SR 1 Lucas or even the Picador type Scintilla.
As a point of clarification, those are of the rotating magnet, not armature, type.

I have a 400 MHz 4-channel oscilloscope and the necessary high frequency, current and high voltage probes to simultaneously measure the primary and secondary circuits of a magneto. But, recording the four sets of I and V data manually while changing the rotation speed of a lathe (and accurately measuring it using a remote tachometer) is a lot of work, after which manually calculating the spark energy vs. rpm requires even more work. It's enough work that I've only done it for one magneto, and even then only over a limited range. This is why I designed a comprehensive tester, to automate everything automate-able in order to measure whatever I want without it being a major production to set up and implement each time.
 

bmetcalf

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Perhaps a little OT, but here is the mag for a 1910 Chase (2 stroke)
 

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